The quality of electronic grade glass substrates is evaluated in terms of the size and population of defects on the substrate, flatness, surface roughness, photochemical stability of material, and surface chemical stability. The trend toward higher accuracy of the design rule poses ever-tightening requirements on these factors.
Photomask substrates used in the fabrication of microelectronic devices must have a high accuracy of shape or topography. If a substrate has a poor accuracy of shape or a certain degree of distortion, exposure through the mask entails a focal shift on a silicon wafer to aggravate pattern uniformity, failing to form a fine feature pattern. In the lithography using ArF laser light source of 193 nm wavelength which is the current mainstream of microelectronic lithography and the EUV lithography using a light source of 13.5 nm wavelength in the soft x-ray wavelength range on which development efforts are made as the next generation lithography, glass substrates for photomasks and reflection masks are required to have a high level of topographical factors including flatness, parallelism, and contour tolerance (as to flatness, see JP-A 2008-103512). The same applies to photomask substrates on the array side of TFT liquid crystal panels and photomask substrates for color filters.
Research works have also been made on the nanoimprint lithography (NIL) which is a low cost, simple, high resolution process as compared with the conventional exposure processes. The NIL also needs a substrate having a high accuracy of shape as the imprint mold. The NIL is a technique of imprinting a nano-structured pattern to a resin for pattern transfer. The resolution of the pattern to be transferred depends on the resolution of nano-structures on the mold. Then the substrate on which a fine feature pattern is formed is required to have a high accuracy of shape (see JP-A H03-54569).
Besides, the synthetic quartz glass members to be assembled in exposure tools and various other equipment used in the processes of manufacturing microelectronic components are also required to have a high purity and accuracy.